Frequency-Selective Surfaces Go in Circles

A frequency-selective surface (FSS) holds great promise for a wide range of applications, from communications and security systems through radomes and radar surfaces. An FSS can function as a combination antenna, filter, and shield, depending on the shape of the FSS. Although not traditionally used, researchers from Spain and Portugal explored using combinations of multiple semicircles in two different FSS designs: a single-band, band-rejection design (or single-band, bandpass design when configured in reverse) and a dual-band FSS.

The single-band/band-reject FSS design was obtained from four semicircles with a 90-deg. rotation around the center between each other. The dual-band FSS combines eight semicircles in a pattern resembling a square shape, with alternating 270 and 180 deg. rotation around the edge of each semicircle. Such parameters as the conductor thickness and the radius of each semicircle determine the resonant frequencies of the structures.

Both FSS designs were simulated with commercial electromagnetic (EM) simulation software—CST Microwave Studio from Computer Simulation Technology. The four-semicircle FSS design was designed for resonant frequency of 2.4 GHz and compared with classical FSS designs using square, ring, and swastika configurations.

The four semicircles provided a bandwidth of 330 MHz at 2.4 GHz, compared to bandwidths of 1140, 820, and 680 MHz at 2.4 GHz for the square, ring, and swastika FSS configurations. Similarly, the eight-semicircle FSS was compared with dual-band classical FSS designs. The first resonance at about 3.1 GHz had a bandwidth of 600 MHz; the second resonance, at about 7.2 GHz, had a bandwidth of 275 MHz, or considerably narrower than the earlier dual-band FSS designs.

Measurements on the two FSS designs were made by fabricating the semicircle configurations in large numbers on FR-4 printed-circuit-board (PCB) material and then characterizing the PCBs on commercial test equipment. The single-band FSS was tuned to 2.4 GHz while the dual-band FSS was tuned for both 2.37 and 5.29 GHz. Relatively good agreement between computer simulations and measured results was found when allowing for small deviations in fabricated FSS dimensions.